A feed-water pump for a small once-through boiler is usually controlled to start an operation when a boiler water level reduces down to a setting lower limit value or below, and to stop the operation when the boiler water level reaches a setting upper limit value. Accordingly, the feed-water pump irregularly operates and causes variations in a flow rate of feed-water depending on a combustion state of the boiler. In the case where a plurality of small once-through boilers are installed and operated, feed-water pumps for the individual boilers irregularly operate and cause variations in flow rates of feed-water. Therefore, a flow rate of the feed-water in a feed-water main pipe varies to a large extent.
Injection of a chemical liquid into the small once-through boiler is generally controlled in a manner using a microcomputer, an ON/OFF signal from the feed-water pump, or a pulse signal or an analog signal from a flowmeter.
The control using a microcomputer is executed by assuming that feed-water has been supplied to the boiler in an amount depending on a combustion state and a combustion time of the boiler, and by determining an operation time of a chemical injection pump corresponding to the amount of the supplied feed-water. In the microcomputer control, the chemical injection pump is not always in conjunction with the operation of the feed-water pump. In some cases, the chemical injection pump is operated when the feed-water pump is not operated, and conversely the chemical injection pump is not operated even when the feed-water pump is operated. Hence a chemical concentration in the feed-water tends to greatly vary in the microcomputer control.
In the manner of controlling the chemical injection pump in accordance with an ON/OFF signal from the feed-water pump, variations of the chemical concentration in the feed-water are smaller than those in the case of the microcomputer control, because the chemical injection pump is operated when the chemical injection pump is under the operation. However, since a flow rate of feed-water is slightly reduced just after startup of a feed-water pump and at the end of the started operation, an injection amount of a chemical (dosage) is relatively large just after the startup and at the end of the started operation.
In the manner of controlling the chemical injection pump in accordance with a pulse signal or an analog signal from a feed-water flowmeter, the chemical concentration in the feed-water can be held close to a target concentration, because the number of strokes per minute of the chemical injection pump is determined in proportion to the flow rate of the feed-water. In the case of supplying the feed-water by merely controlling the chemical injection pump in accordance with the pulse signal or the analog signal from the feed-water flowmeter, an error between an actual injection amount and a target injection amount increases when the performance of the feed-water pump or the chemical injection pump degrades. If the chemical concentration in the feed-water is lower than the target concentration, there arise problems, for example, that the effect of an anticorrosive becomes insufficient in equipment (such as a feed-water pipe and an economizer) prior to reaching a boiler drum, and that the deoxidization effect of a deoxidizer reduces. Conversely, if chemicals are added in excess, it is uneconomical.
Japanese Patent 4390473 (Japanese Patent Publication 2004-321860A) proposes a correction method of detecting an injection amount by a flowmeter installed in a chemical injection pipe, and calculating a feedback value on the basis of a detected value in real time. However, the proposed method is to feedback-control a pumping rate of the chemical injection pump from a measured value of a flow rate when change of the flow rate is small. Hence the proposed method has a problem that, in a facility causing large variations in the flow rate as in a feed-water facility equipped with a plurality of small once-through boilers, a control signal varies to a large extent depending on the timing of detection by the flowmeter, time lags in calculation and control, etc., and a difficulty occurs in keeping the injection amount appropriate.
Usually, one or two types of multifunction boiler cleaners in combination of plural kinds of chemicals having water treatment effects are injected to a low-pressure boiler. In some cases, however, a particular component becomes deficient or excessive for the reason that the quality or the temperature of feed-water varies due to variations in drain recovery or quality of raw water, and that balance among the required amounts of an alkali aid, a deoxidizer, an anticorrosive, a dispersant, etc. varies.
An anticorrosive component is controlled by a method on the basis of the concentration of the anticorrosive component added to the feed-water, or a method on the basis of the concentration of the anticorrosive component in a boiler water. In the former method, the chemical (anticorrosive component) is injected to keep constant the chemical concentration in the feed-water regardless of the drain recovery. When the latter method is applied to a boiler in which a degree of concentration is controlled by discharging blow water upon the electrical conductivity of the boiler water reaching a setting upper limit value, and by stopping the discharge of the blow water upon reaching a setting lower limit value, the following problem arises. Because the concentration degree varies when the electrical conductivity of the feed-water varies due to variations in the drain recovery rate or the quality of the feed-water, the chemical concentration in the boiler water cannot be controlled at a constant level even when the chemical injection is performed in proportion to the flow rate of the feed-water.
It has been difficult to keep the boiler water treatment optimum due to the complicated actions of the above-described problems, i.e., the problem that the injected chemical concentration with respect to the feed-water cannot be maintained, and that the required injected chemical concentration varies depending on operating conditions of the boiler. If the chemical concentration is lower than a target value, the water treatment effect becomes insufficient in some cases. For example, the effect of the anticorrosive is insufficient and corrosion progresses. The deoxidization effect of the deoxidizer reduces, and a large amount of oxygen transits into vapor, thus generating corrosion in a condensate pipe. On the other hand, if the chemicals are added in excess, the electrical conductivity of the boiler water excessively increases to such an extent as causing carryover, or the chemical cost rises.
Methods for controlling a target injection amount, i.e., a discharge rate of a chemical injection pump, depending on variations in drain recovery or quality of raw water have been proposed as below. Japanese Patent Publication 2010-159965A discloses a method of determining an operation time of a chemical injection pump for a pH adjuster depending on a concentration of carbonic acid in feed-water. Japanese Patent Publication H10-82503A discloses a method of determining operation time of a chemical injection pump depending on a temperature of feed-water. However, any of those methods is not adaptable for change of a discharge rate due to, e.g., the above-described reduction in the performance of a chemical injection pump or a feed-water pump.
When a chemical is injected to feed-water such that a concentration thereof in the feed-water is constant, a period-average injected chemical concentration can be manually checked by recording a chemical reduction amount and an integrated value of the feed-water flow rate. However, when feed conditions of a chemical injection pump are changed depending on variations in drain recovery or a quality of raw water, a reduction amount of a chemical and an integrated value of a feed-water flow rate have to be checked each time the feed conditions are changed. It is not realistic from the practical point of view to change the feed conditions of the chemical injection pump while recognizing usage situations of vapor in a process, and variations in the drain recovery or the quality of the raw water at all times, which are affected by weather, air temperature, etc. Thus, whether the injected chemical concentration is controlled as per a target has been infeasible to check.
Patent Literature 1: Japanese Patent 4390473
Patent Literature 2: Japanese Patent Publication 2010-159965A
Patent Literature 3: Japanese Patent Publication H10-82503A